The present invention relates to optical imaging devices and imaging methods for microscopy. The invention can be applied in connection with the inspection of arbitrary surfaces or bodies.
In many technical areas, it is necessary, amongst other, to subject bodies and their surfaces to a precise optical inspection in order to be able, for example, to assess the quality of a production process and, where applicable, intervene correctively insofar as the inspection reveals that specified quality criteria are not fulfilled. Naturally the same if not higher requirements must be imposed on the precision of the imaging device used for the inspection in comparison with the devices used for the production process of the body to be inspected.
In this context, the ability of the imaging device used for the inspection to process light of different wavelengths with minimum optical aberration is of particular importance in order to ensure a broad field of application for the imaging device. In connection in particular with the production methods which comprise an optical process, it is desirable or advantageous if the imaging device used can process the wavelength range typically also used during the optical process with minimum aberrations. This is preferably the wavelength range of 193 mm (so called VUV range) to 436 mm (so called Hg g line). Such broad bandwidth requirements exist, for example, in the field of fluorescence microscopy in connection with the avoidance of thin layer interference on the surface etc.
A problem here is the chromatic aberrations i.e. the aberrations dependent on the light wavelength. If for example an imaging device with refractive optical elements (such as lenses or similar) is used for inspection, the aberrations of the imaging device are minimized at acceptable cost usually only for a comparatively narrow range of wavelengths. A so-called achromatization of such an imaging device comprising refractive optical elements, i.e. elimination of such chromatic aberrations, is scarcely possible with acceptable cost over a broadband wavelength range (such as that described above).
Frequently, so-called catadioptric imaging devices are used which, apart from refractive optical elements, also comprise reflective optical elements. The disadvantages of refractive systems described above however also apply to such catadioptric systems as known for example from DE 10 2005 056 721 A1 (Epple et al.), U.S. Pat. No. 6,600,608 B1 (Shafer et al.), U.S. Pat. No. 6,639,734 B1 (Omura) and U.S. Pat. No. 5,031,976 (Shafer), the entire disclosure of which is hereby incorporated herein by reference.
One possibility of largely avoiding the problems associated with chromatic aberrations is to use so-called catoptric systems in which exclusively reflective optical elements (such as mirrors or similar) are used for the imaging device. Examples of such catoptric systems are known from EP 0 267 766 A2 (Phillips), U.S. Pat. No. 4,863,253 (Shafer et al.) and US 2004/0114217 A1 (Mann et al.), the entire disclosure of which is hereby incorporated herein by reference.
The problem with these known catoptric systems however is that, for a desirably large magnification to be achieved with as few optical elements as possible, in particular for the optical elements close to the object, comparatively large individual refractive powers are required. However, in view of the aberrations generated with such catoptric systems, this is disadvantageous so that frequently the use of more than four mirrors is preferred, as is known from US 2004/0114217 A1 (Mann et al.), or smaller magnifications or greater aberrations, respectively, are accepted.
In this context it is known from EP 0 267 766 A2 (Phillips), instead of the conventional imaging devices with four mirrors, to use a system with three mirrors in which one of the mirrors is used repeatedly in that it constitutes both the second mirror and the fourth mirror in the path of the imaging beam. The advantage of this is that one mirror is saved. The arrangement of the other two mirrors which must each face this mirror requires, however, a configuration with which only a comparatively small numerical aperture can be achieved (at acceptable mirror size).